Temperature-/solvent-dependent low-dimensional compounds based on quinoline-2,3-dicarboxylic acid: Structures and fluorescent properties

Article abstract of DOI:10.1039/c2dt31243c

A series of 0-D, 1-D, and 2-D metal-organic compounds through reactions of quinoline-2,3-dicarboxylic acid (2,3-H₂qldc) with transition metal salts MCl₂, namely, M(2,3-Hqldc)₂(H₂O) ₂ (M = Co(1), Zn(4) and Cd(7)), [M(3-qlc)₂(H ₂O)₂]_n (M = Co(2), Zn(5) and Cd(8)), M(2-qldc-3-OCH₃)₂(CH₃OH)₂ (M = Co(3) and Zn(6)) and [Cd(2,3-qldc-OCH₃)(μ₂-Cl)]_2n (9) (where, 3-Hqlc = quinoline-3-carboxylic acid and 2-qldc-3-OCH₃ = 3-(methoxycarbonyl)quinoline-2-carboxylic acid), were synthesized and characterized by elemental analysis, IR, thermogravimetric analysis (TG), and single-crystal X-ray diffraction. When the temperature ranged from room temperature to 70 °C, three isomorphous mononuclear complexes 1, 4 and 7 were obtained in H₂O/H₂O + CH₃OH. As the temperature rose further to above 90 °C, due to the decomposition of 2-position carboxyl group in ligand 2,3-H₂qldc, the same reactions, respectively, produced three isomorphous 2-D layer-like structures 2, 5 and 8 with 4⁴ topology in water. By contrast, when the mixed solvent of H₂O + CH₃OH at a 1:1 ratio (v/v) was applied, the three above-mentioned reactions respectively gave compounds 3, 6 and 9 with the 3-position esterification of 2,3-H₂qldc. Compounds 3 and 6 are mononuclear and isomorphous, while complex 9 has a 1-D double-stranded chain-like structure connected by two μ₂-Cl bridges. Obviously, these results reveal that the reaction temperature and solvent play a critical role in structural direction of these low-dimensional compounds. Meanwhile, the photoluminescent property of the selected compounds is also investigated.

Full text of DOI:10.1039/c2dt31243c

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PAPER
Temperature-/solvent-dependent low-dimensional compounds based on
quinoline-2,3-dicarboxylic acid: Structures and fluorescent properties†
Ming-Fang Wang,a Xu-Jia Hong,a Qing-Guang Zhan,a Hong-Guang Jin,a Yi-Ting Liu,a Zhi-Peng Zheng,a
Shi-Hai Xub and Yue-Peng Cai*a
Received 9th June 2012, Accepted 4th August 2012
DOI: 10.1039/c2dt31243c
A series of 0-D, 1-D, and 2-D metal–organic compounds through reactions of quinoline-2,3-dicarboxylic
acid (2,3-H2qldc) with transition metal salts MCl2, namely, M(2,3-Hqldc)2(H2O)2 (M = Co(1), Zn(4) and
Cd(7)), [M(3-qlc)2(H2O)2]n (M = Co(2), Zn(5) and Cd(8)), M(2-qldc-3-OCH3)2(CH3OH)2 (M = Co(3)
and Zn(6)) and [Cd(2,3-qldc-OCH3)(μ2-Cl)]2n (9) (where, 3-Hqlc = quinoline-3-carboxylic acid and
2-qldc-3-OCH3 = 3-(methoxycarbonyl)quinoline-2-carboxylic acid), were synthesized and characterized
by elemental analysis, IR, thermogravimetric analysis (TG), and single-crystal X-ray diffraction. When
the temperature ranged from room temperature to 70 °C, three isomorphous mononuclear complexes 1, 4
and 7 were obtained in H2O/H2O + CH3OH. As the temperature rose further to above 90 °C, due to the
decomposition of 2-position carboxyl group in ligand 2,3-H2qldc, the same reactions, respectively,
produced three isomorphous 2-D layer-like structures 2, 5 and 8 with 44 topology in water. By contrast,
when the mixed solvent of H2O + CH3OH at a 1 : 1 ratio (v/v) was applied, the three above-mentioned
reactions respectively gave compounds 3, 6 and 9 with the 3-position esterification of 2,3-H2qldc.
Compounds 3 and 6 are mononuclear and isomorphous, while complex 9 has a 1-D double-stranded
chain-like structure connected by two μ2-Cl bridges. Obviously, these results reveal that the reaction
temperature and solvent play a critical role in structural direction of these low-dimensional compounds.
Meanwhile, the photoluminescent property of the selected compounds is also investigated.
during research is whether the topology of coordination poly-
mers and metal–organic frameworks solely based on the molecu-
Introduction
Coordination polymers (CPs) or metal–organic frameworks
(MOFs) involving the fields of crystal engineering and supra-
molecular chemistry represent a rapidly growing subject area
over the last two decades, not only because of the enormous
variety of interesting molecular topologies from the bridging
potential of different organic linkers and geometry of the metal
ions, which are often unprecedented in inorganic compounds
and minerals,1 but also due to their excellent properties with
promising applications such as gas storage, gas/vapor separation,
size-, shape-, and enantio-selective catalysis, luminescence and
fluorescence, drug storage and delivery.2 Due to the increasing
interest in coordination polymers, an issue frequently confronted
lar building blocks can be predicted. And the answer at this
moment is “yes, we can, to a certain extent”. In fact, the proper
choice of metal and organic linker ligand can already put a limit
on the number of possible network architectures. Other external
factors, such as type of anion, metal-to-ligand ratio, solvent,
temperature, and pH also have an influence on the formation,
connectivity, and topology of coordination polymers.3 Among
these influencing factors, the variation of reaction temperature
and solvents, in particular, plays a very important role in the
self-assembly processes of metal-complexes with different struc-
tural topologies.4 For example, recently, Brookhart et al.4a exhib-
ited a highly solvent- and temperature-dependent equilibrium in
solution between a four coordinated Ir(I) dihydrogen species and
an Ir(III) dihydride which incorporates solvent coordination on
the basis of a series of NMR spectroscopic deuterium labeling
studies. And Zhang et al.5 also showed the effect of temperature
and solvent on the morphology of microcapsules doped with a
europium β-diketonate complex. In our recent work, temp-
erature- and solvent-effects have also received much attention in
the construction of metal–organic polymers.6 Such temperature-
and solvent-effects are now an important and effective route in
constructing metal–organic compounds with diverse topologies
and dimensionality, and so on.
aSchool of Chemistry and Environment, South China Normal
University; Key Laboratory of the Energy Conversion and Energy
Storage Materials, Guangzhou 510006, P.R. China.
E-mail: caiyp@scnu.edu.cn; Fax: +86-020-39310;
Tel: +86-020-39310383
bDepartment of Chemistry, Jinan University, Guangzhou 510632,
P.R. China. E-mail: txush@jnu.edu.cn
†Electronic supplementary information (ESI) available. CCDC 849592,
849594, 849593, 849595, 849598, 849596, 849597, 849600 for com-
pounds 1–6 and 8–9, respectively. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c2dt31243c
11898 | Dalton Trans., 2012, 41, 11898–11906
This journal is © The Royal Society of Chemistry 2012

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On the other hand, the N-heterocyclic dicarboxylate ligands,
for example pyridine-dicarboxylic acids (H2pydc), have attracted
increasing attention in the preparation of interesting polymeric
frameworks for their outstanding features of versatile coordi-
nation fashions as well as potential hydrogen-bonding donors
and acceptors under hydro(solvo)thermal conditions.7 Accord-
ingly, pyridine-dicarboxylic acids (H2pydc) are excellent candi-
dates for assembling novel CPs/MOFs by incorporating
appropriate metal ions in different ways, based on which many
CPs/MOFs possessing beautiful and interesting topological
structures are reported by our and other research groups,8
ranging from one-dimensional (1-D) chains and two-dimensional
(2-D) sheets to three-dimensional (3-D) porous structures. Of
further interest, quinoline-2,3-dicarboxylic acid (2,3-H2qldc) fea-
turing a fused benzene ring in the 5,6-positions of the pyridine
ring, as a derivative of 2,3-H2pydc, remains largely unexplored
hitherto in the field of CPs/MOFs compared with the
well-studied ligands H2pydc. 2,3-H2pydc has a relatively large
π-conjugated system in the quinoline ring, which might not only
contribute much to the desirable fluorescence properties resulting
from the interaction between 2,3-qldc2− anions and metal ions,
but also easily assemble into high dimensionality (2-D or 3-D)
supramolecular networks via π⋯π packing interactions between
two adjacent aromatic rings as well as hydrogen bonding
C–H⋯π interactions, adding additional stability to these struc-
tures. Based on the above considerations, we hope to reveal
some structural factors of the ligand 2,3-H2qldc in dominating
the self-assembly, which will provide more useful information of
the temperature- and solvent-effects in such pyridine-based
dicarboxylate ligands.
dried by standard procedures. Elemental analyses for C, H, N
were performed on a Perkin-Elmer 240C analytical instrument.
IR spectra were recorded on a Nicolet FT-IR-170SX spectro-
photometer in KBr pellets. Thermogravimetric analyses were
performed on a Perkin-Elmer TGA7 analyzer with a heating rate
of 10 °C min−1 in a flowing air atmosphere. The luminescent
spectra for the solid state were recorded at room temperature on
an Edinburgh-FLS-920 instrument with a xenon arc lamp as the
light source. In the measurements of emission and excitation
spectra the pass width is 5.0 nm.
Preparation of complexes 1–9
A mixture of MCl2 (0.3 mmol) (M = Co for 1, Zn for 4, Cd for
7), ligand 2,3-H2qldc (44 mg, 0.2 mmol) and H2O (12 mL) was
heated at room temperature to 70 °C for 80 h in a 25 mL Teflon-
lined stainless-steel autoclave and then cooled to room temp-
erature at a rate of 10 °C h−1 to obtain the corresponding crystals.
When the temperature exceeded 90 °C, the three reactions
gave three partially decarboxylated compounds 2, 5 and 8 corre-
spondingly. By contrast, when the mixed solvent of H2O +
CH3OH at a 1 : 1 ratio (v/v) was used, the three above-mentioned
reactions gave the partially esterified compounds 3, 6 and 9,
respectively.
1. Yield 65% (based on the Co), yellow crystals. Elemental
analysis calcd (%) for C22H16N2O10Co: C,50.07; H,3.06;
N,5.34. Found: C,50.10; H,3.08; N,5.31. FT-IR (KBr, cm−1):
3367(br,vs), 1728(vs) 1678(vs), 1554(vs), 1458(vs), 1373(s),
1273(w), 1184(m), 1157(m), 910(m), 856(m), 775(m), 748(w),
682(w), 536(w), 459(w).
As an extension of our recent efforts to explore coordination
chemistry and crystal engineering of pyridine-based dicarboxy-
late ligand, in this contribution, we prepared a series of low-
dimensional compounds with variable topologies based on qui-
noline-based dicarboxylate. The syntheses, structural analysis,
thermal stability, and photoluminescent properties of nine CoII,
ZnII and CdII complexes, namely, [M(2,3-Hqldc)2(H2O)2] (M =
Co(1), Zn(4) and Cd(7)), [M(3-qlc)2(H2O)2]n (M = Co(2), Zn(5)
and Cd(8)), M(2-qldc-3-OCH3)2(CH3OH)2 (M = Co(3) and
Zn(6)) and [Cd(2,3-qldc-OCH3)(μ2-Cl)]2n (9) (where 3-Hqlc =
quinoline-3-carboxylic acid, and 2-qldc-3-OCH3 = 3-(methoxy-
carbonyl)quinoline-2-carboxylic acid), are presented. Our study
shows that the variation of temperature and solvents in a Co2+/
Zn2+/Cd2+ quinoline-2,3-dicarboxylate system can influence
subtle variables that lead to coordination compounds with differ-
ent structures. Especially, the temperature-driven decarboxylation
and the temperature/solvent-induced esterification of the car-
boxyl group can control the assembly of the resulting metal–
organic compounds (Scheme 1). To the best of our knowledge,
this is the first example of a systematic investigation into the
coordination chemistry as well as temperature- and solvent-
effects in the system of quinoline-2,3-dicarboxylic acid.
2. Yield 49% (based on the Co), yellow crystals. Elemental
analysis calcd (%) for C20H12N2O4Co: C,59.52; H,2.98; N,6.94.
Found: C,55.48; H,2.97; N,7.01. FT-IR (KBr, cm−1): 3413(br,
w), 1633(vs), 1595(m), 1463(vs) 1382(vs), 1282(s), 1209(s),
1152(s), 938(m), 897(w), 789(m), 747(m), 664(m), 600(m),
566(m), 488(w).
3. Yield 48% (based on the Co), yellow crystals. Elemental
analysis calcd (%) for C26H24N2O10Co: C,53.48; H,4.11; N,4.8.
Found: C,53.55; H,4.13; N,4.68. FT-IR (KBr, cm−1): 3228(br,
w), 1643(vs), 1558(m), 1458(s), 1377(m), 1249(s), 1176(w),
1138(w), 906(w), 856(m), 798(m), 783(m), 667(w), 621(w),
493(w), 455(w).
4. Yield 51% (based on the Zn), colorless crystals. Elemental
analysis calcd (%) for C22H16N2O10Zn: C,50.07; H,2.71;
N,4.75. Found: C,50.17; H,2.80; N,4.70. FT-IR (KBr, cm−1):
3367(br,vs), 1728(vs) 1678(vs), 1554(vs), 1458(vs), 1373(s),
1273(w), 1184(m), 1157(m), 910(m), 856(m), 775(m), 748(w),
682(w), 536(w), 459(w).
5. Yield 71% (based on the Zn), colorless crystals. Elemental
analysis calcd (%) for C20H12N2O4Zn: C,58.58; H,2.93; N,6.83.
Found: C,58.47; H,2.99; N,6.73. FT-IR (KBr, cm−1): 3413(br,
w), 1633(vs), 1595(m), 1463(vs), 1382(vs), 1282(s), 1209(s),
1152(s), 938(m), 897(w), 789(m), 747(m), 664(m), 600(m),
566(m), 488(w).
Experimental
6. Yield 50% (based on the Zn), colorless crystals. Elemental
analysis calcd (%) for C26H24N2O10Zn: C,50.07; H,2.71;
N,4.75. Found: C,50.17; H,2.80; N,4.70. FT-IR (KBr, cm−1):
3228(br,w), 1643(vs), 1558(m), 1458(s), 1377(m), 1249(s),
Physical measurements
All materials were reagent grade obtained from commercial
sources and used without further purification, and solvents were
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Scheme 1 Assembly of compounds 1–9 controlled by temperature and solvents.
1176(w), 1138(w), 906(w), 856(m), 798(m), 783(m), 667(w),
621(w), 493(w), 455(w).
hydrogen atoms were refined anisotropically. The hydrogen
atoms were placed at calculated positions and included in the
refinement in the riding model approximation. The organic
hydrogen atoms were generated geometrically (C–H = 0.93 or
0.96 Å), and the water hydrogen atoms were located from differ-
ence maps and refined with isotropic temperature factors. Details
of the crystal parameters, data collections, and refinements for
complexes 1–6 and 8–9 are summarized in Table S1.† Selected
bond lengths and angles for eight complexes are shown in
Table S2,† hydrogen-bonding data of these eight complexes are
listed in Table S3 (ESI†). Further details are provided in the
ESI.† Crystallographic data for the structures reported in this
paper have been deposited with the Cambridge Crystallographic
Data Centre.
7. Yield 67% (based on the Cd), colorless crystals. Elemental
analysis calcd (%) for C22H16N2O10Cd: C,50.07; H,3.06;
N,5.34. Found: C,55.17; H,3.08; N,5.31. FT-IR (KBr, cm−1):
3367(br,vs), 1725(w), 1678(vs), 1554(vs), 1458(vs), 1373(s),
1273(w), 1184(m), 1157(m), 910(m), 856(m), 775(m), 748(w),
682(w), 536(w), 459(w).
8. Yield 66% (based on the Cd), colorless crystals. Elemental
analysis calcd (%) for C20H12N2O4Cd: C,52.55; H,2.63; N,6.13.
Found: C,52.47; H,2.68; N,6.11. FT-IR (KBr, cm−1): 3413(br,
w), 1633(vs), 1595(m), 1463(vs), 1382(vs), 1282(s), 1209(s),
1152(s), 938(m), 897(w), 789(m), 747(m), 664(m), 600(m),
566(m), 488(w).
9. Yield 49% (based on the Cd), colorless crystals. Elemental
analysis calcd (%) for C12H8NClO4Cd: C,38.10; H,2.12; N,3.70.
Found: C,38.17; H,2.18; N,3.64. FT-IR (KBr, cm−1): 3363(br,
w), 1643(vs), 1546(m), 1454(vs), 1384(s), 1346(s), 1207(m),
1141(s), 1056(m), 906(w), 852(m), 817(m), 775(m), 748(m),
671(m), 594(m), 543(w), 474(w).
Results and discussion
Synthetic and spectral aspects
Through conventional solution reaction or hydro(solvo)thermal
syntheses, nine new compounds 1–9 were synthesized and
characterized by EA, IR and X-ray single-crystal diffraction.
Although compound 7 has no single crystal structure, the results
of its EA and TG confirm that 7 and 1, 4 are isomorphous.
When the reaction medium is water, the major ligand 2,3-H2qldc
reacting with MIICl2 easily lost one 2-position carboxyl group
under high temperature (for 2, 5 and 8). However, when the
mixture solvent of H2O + CH3OH is used, due to partial esterifi-
cation of ligand 2,3-H2qldc at high temperature, three com-
pounds 3, 6 and 9 were obtained, indicating temperature and
X-ray data collection and structure refinement
Data collections were performed at 298 K on a Bruker Smart
Apex II diffractometer with graphite-monochromated MoKα
radiation (λ = 0.71073 Å) for 1–6, and 8–9. Absorption correc-
tions were applied by using the multiscan program SADABS.9
Structural solutions and full-matrix least-squares refinements
based on F2 were performed with the SHELXS-9710 and
SHELXL-9711 program packages, respectively. All the non-
11900 | Dalton Trans., 2012, 41, 11898–11906
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solvent have a certain effect on the formation of these metal–
organic compounds in the 2,3-H2qldc system.
distance is 2.228(4) Å. Only one of the two carboxyl groups in
ligand 2,3-H2qldc was deprotonated and adopts the μ-η1:η1
coordination fashion (mode I in Scheme 2). In addition to intra-
molecular hydrogen bonds O(C)–H⋯O, the intermolecular
hydrogen bond O–H⋯O also plays an important role in assem-
bling the mononuclear molecule Co(2,3-Hqldc)2(H2O)2 into 2-D
supramolecular network (Fig. 2). From Fig. 2a, each neutral
mononuclear unit forms a series of acceptor/donor hydrogen
bonds with four neighbouring ones via intermolecular hydrogen
bonds involving O–H⋯O, leading to the formation of a two-
dimensional ordered layer-like structure. If the mononuclear unit
is viewed as the node and the hydrogen bonding O–H⋯O inter-
action from four neighbouring ones as linkers (Fig. 2b), the
resulted 2D supramolecular structure may be simplified into a
two-dimensional 44 rhombic-grid as depicted in Fig. 2b.
The IR spectra of 1–9 are similar. The strong and broad
absorption bands in the range of 3100–3600 cm−1 of these com-
pounds may be assigned to the characteristic peaks of νO–H
stretching vibrations from coordinated water or methanol mole-
cules. The strong vibrations appearing around 1635, 1590, and
1450 cm−1 correspond to the asymmetric and symmetric stretch-
ing vibrations of the carboxyl group, respectively.12 The absence
of strong bands ranging from 1690 to 1730 cm−1 for complexes
2, 3, 5, 6, 8 and 9 indicates that the ligand in these complexes
are deprotonated, compared with those of free ligands in these
complexes.
Thermal stability of the complexes was investigated by the
TGA technique (see Fig. S1, ESI†). The isomorphous phenom-
enon exists in nine compounds 1–9, so only four compounds 1,
2, 3 and 9 were selected to examine their thermal stability. The
TGA curve of compound 1 shows that the weight loss of 7.10%
around 115 °C attributes to the removal of coordinated water
(calc. 6.83%). Then the framework remains integrity until
261 °C (Fig. S1a†). The TGA curve of 2 shows the complex is
very steady and there is no weight loss until 293 °C (Fig. S1b†).
The TGA curve of 3 shows continuous weight losses in the
temperature range of 122–167 °C are because of the removal of
coordinated methanol molecules. The observed weight loss of
10.15% is in good agreement with the calculated value
(10.97%). Then the framework begins to decompose when the
temperature rise up to 226 °C (Fig. S1c†). The TGA curve of
compound 9 shows the complex begins to lose weight up to
270 °C and then begins to decompose (Fig. S1d†).
[MII(3-qlc)2(H2O)2]n (M = Co(2), Zn(5) and Cd(8)). When
heated up to 90–160 °C, the reactions of 2,3-H2qldc with MCl2
gave three isostructural complexes 2 (M = Co), 5 (M = Zn) and
8 (M = Cd). Considering the isomorphous phenomenon, only
the structure of complex 2 is described in detail. Single crystal
X-ray diffraction shows that complex 2 crystallized in the mono-
clinic space group C2/c, in which the ligand 2,3-H2qldc became
3-qlc− via 2-position decarboxylation. As shown in Fig. 3, the
Co1 is five coordinated to three O atoms from two 3-qlc− and
two N atoms from two other 3-qlc−, and exhibits the coordi-
nation geometry of a distorted square-pyramid around the
cobalt(II) ion with the N2 atom occupying the apical position
and O1, N1, O2, O3 defining the basal plane. The Co–O dis-
tances are 1.936(2) and 1.977(2) Å and the Co–N distances are
2.085(2) and 2.103(2) Å. Every ligand 3-qdc− in 2, in which
ligand 3-qdc− adopts two coordination manners, namely mode
II: μ2-η1:η1 and mode III: μ2-η1:η1:η1 in Scheme 2, links two
Co2+ ions using one/two O atoms from carboxyl and one N atom
to form a 2D layer-like structure (Fig. 4a). When we consider the
CoII as the node and the ligand as the linker, the simplified topo-
logical representation of compound 2 can be described in
Fig. 4b, which exhibits a 4-connected 44 topology with one
wave side.
Structural analysis and discussion
M(2,3-Hqldc)2-(H2O)2 (M = Co(1) and Zn(4)). The 0-D
mononuclear coordination compounds 1 and 4 are isostructural
ˉ
and crystallize in the triclinic space group P1. Here, we choose 1
to represent the detailed structure. The mononuclear molecule
contains one CoII cation, two partially deprotonated 2,3-Hqldc−
anions and two coordinated water molecules. The coordination
number of Co1 is six with two nitrogen and four oxygen atoms
from two individual ligands 2,3-qldc− and two coordinated
water molecules. The coordination geometry of the Co(II) center
is a slightly distorted octahedron as shown in Fig. 1. The Co–O
distances are in the range of 2.027(3)–2.093(3) Å and the Co–N
Moreover, inter-layer π⋯π stacking interaction with the dis-
tance of 3.761 Å between the centers of two benzene/pyridine
rings, links the adjacent coordination layers to form a 3-D
network (Fig. 5).
M(2,3-qldc-OCH3)2(CH3OH)2 (M = Co(3) and Zn(6)). As the
control, when the mixed solvent of H2O + CH3OH at a 1 : 1 ratio
(v/v) was applied, the reaction of 2,3-H2qldc with MCl2 gave
three compounds 3, 6 and 9 with the 3-position esterification of
2,3-H2qldc. Single crystal X-ray diffraction shows that two com-
plexes 3 and 6 are isostructural and crystallize in the monoclinic
space group P2(1)/c. Similarly, compound 3 is selected to rep-
resent the two structures. In 3, as shown in Fig. 6, the mono-
nuclear molecule contains one CoII centers, two esterified 2,3-
(qldc-OCH3)− units and two coordinated methanol molecules.
The Co1 is six-coordinated by two nitrogen and four oxygen
atoms from two esterified individual ligands 2,3-(qldc-OCH3)−
and two coordinated methanol molecules. The Co–O distances
are in the range of 2.008(3)–2.125(4) Å and the Co–N distance
is 2.218(4) Å. Because one of the two carboxyl groups in ligand
Fig. 1 The coordination environments of the CoII ion in compound 1
containing intramolecular hydrogen bonds O(C)–H⋯O. Symmetry
code: (a) 1 − x, −y, 1 − z.
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Scheme 2 Coordination modes of 2,3-qldc2−, as well as 2,3-(qldc-OCH3)− and 3-qlc− after partial esterification and decarboxylation in 1–9.
Fig. 2 (a) 2-D supramolecular layer constructed by neutral mono-
nuclear unit Co(2,3-Hqldc)2(H2O)2 via the intermolecular hydrogen
bond O–H⋯O. (b) Two-dimensional 44 rhombic-grid in compound 1.
Fig. 4 2-D wave-like sheet with 44 topology extended in bc plane in 2.
Fig. 3 The coordination environments of the CoII ion in compound 2
containing partial atomic labels. Symmetry code: (a) x, 1 − y, 0.5 + z;
(b) x, −1 + y, z.
Fig. 5 3-D supramolecular network constructed by inter-layer π⋯π
2,3-H2qldc was esterified, the new ligand 2,3-qldc-OCH3 adopts
interaction along b axis in 2.
the μ-η1:η1 coordination fashion (mode IV in Scheme 2). Similar
to compound 1, in addition to an intramolecular hydrogen bond
[Cd(2,3-qldc-OCH3)(μ2-Cl)]2n (9). When warming to above
90 °C, in the mixed solvent of H2O + CH3OH, the reaction of
2,3-H2qldc with CdCl2 gave a 1-D chain-like complex 9, which
is different from the above-discussed mononuclear complexes 3
and 6. Single crystal X-ray diffraction shows that complex 9
crystallized in the monoclinic space group C2/c. The asymmetric
unit contains one crystallographically independent Cd(II) ion,
C–H⋯O, each neutral mononuclear molecule Co(2,3-qldc-
OCH3)2(CH3OH)2 interacts with four neighbouring ones via
intermolecular hydrogen bonding involving O3–H3⋯O2
(Table S2†) leading to the formation of a 2-D 44 supramolecular
network, in which the mononuclear unit is viewed as the node
and the hydrogen bonding O–H⋯O interaction from four neigh-
bouring ones as linkers (Fig. 7).
11902 | Dalton Trans., 2012, 41, 11898–11906
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Fig. 8 Perspective views of the coordination environments of the CdII
ions in compound 9. Symmetry code: (a) x, −y, −0.5 + z; (b) −x, −y,
1 − z.
Fig. 6 The coordination environments of the CoII ion in compound 3
containing partial atomic labels. Symmetry code: (a) 1 − x, 1 − y, 1 − z.
Fig. 9 1-D double-stranded chain connected by two μ2-Cl bridges in
compound 9.
Fig. 7 2-D supramolecular layer with 44 topology constructed by inter-
molecular hydrogen bond O–H⋯O extended in the bc plane in 3.
Fig. 10 3-D supramolecular network constructed by intra-chain π⋯π
stacking interactions along the c axis in 9.
one esterified 2,3-(qldc-OCH3)− unit, and one μ2-Cl− anion
(Fig. 8). Cd2+ ion in compound 9 is six-coordinated by one
nitrogen and three oxygen atoms from two individual ligands as
well as two μ2-Cl atoms, and shows octahedral coordination geo-
metry with O4 and O4a atoms at axial positions and the four
remaining positions are occupied by N1, O3a and two μ2-brid-
ging Cl1, Cl1b in a square plane. The Cd–O distance is in the
range of 2.300(11)–2.472(11) Å and the Cd–N distance 2.395(8) Å.
In this structure, the partially esterified 2,3-(qldc-OCH3)− exhi-
bits the coordination mode V: μ2-η1:η2 as shown in Scheme 2.
The coordinated O4 atom in ligand 2,3-(qldc-OCH3)− adopts a
μ2-mode to bridge two Cd to form a 1-D (–O–Cd–O–)n chain,
and then through two μ2-Cl bridges, the two adjacent (–O–Cd–
O–)n chains are connected to form a 1-D ladder-like chain struc-
ture (Fig. 9). These 1-D chains are further assembled into 3-D
supermolecular layer through inter-layer π⋯π stacking inter-
actions between the adjacent quinoline rings (Fig. 10).
Effect of temperature and solvent on molecular assemblies. In
this article crystal structures of the Co(II)/ Zn(II)/Cd(II)/quinoline-
2,3-dicarboxylate system have been described and discussed,
which have been prepared with either a single-medium of H2O
or a mixed-medium of H2O + CH3OH in the range of room
temperature to 160 °C. From the above description and discus-
sion, it can be seen that different reaction temperatures/solvents
have a great influence on the assembly of these metal–organic
complexes. The overview presented in Scheme 1 shows that via
the hydro-/solvothermal synthesis method in a solvent of H2O/
H2O + CH3OH, 0-D mononuclear compounds (1, 4 and 7) were
always obtained with an integrity of ligand 2,3-H2qldc in the
range of r.t to 70 °C, indicating the solvent seemed to have no
influence on the topology of the final compounds in the present
reaction system at low temperature. With the rise of temperature
from 90 to 160 °C, the reaction between 2,3-H2qldc and MCl2 in
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Scheme 3
However, when the mixed solvent of H2O + CH3OH served as
the reaction medium, after the N atom and the O atom from the
2-position carboxylic group of 2,3-H2qldc coordinated to the
central metal in a chelating fashion, the 3-position carboxylic
group is susceptible to esterification by methanol at high temp-
erature. Compared to the carboxylic group, the coordination
ability of the ester to the metal ion is weaker. Hence coordination
sites of ligand 2,3-(qldc-OCH3)− were reduced after esterifica-
tion, making it very easy to understand the formation of the
resultant mononuclear compounds such as 3 and 6. It is note-
worthy that, in present reaction system, compound 9 is a 1-D
double-stranded chain rather than a 0-D mononuclear compound.
An explanation for this observation has not yet been found, but
it certainly has to do with the cation, since this is the only differ-
ence among the three compounds 3, 6 and 9. Cd2+ ion has the
largest ionic radius among the three ions involved Co2+(3),
Zn2+(6) and Cd2+(9), which makes it possible for the counter-
anion Cl− to further coordinate to Cd2+ ion in a μ2-bridging
fashion, resulting in the formation of the 1-D chain in compound
9. As indicated above, obviously directing coordination of these
quinoline-involved ligands to metal centers is the synergy of
various factors, but the temperature and solvent indeed play an
important role in inducing the arrangement and coordination
modes of the ligands, subsequently leading to different topologi-
cal structures of metal–organic complexes in the present reaction
system and under the reaction conditions employed (Scheme 3).
Fig. 11 Luminescent behaviors of compounds 4–9 in the solid state at
room temperature.
H2O gave 2-D 44 isostructural compounds 2, 5 and 8 with the
decarboxylation of 2-position carboxyl group in 2,3-H2qldc,
which suggests that the temperature plays an important role in
the present case. While the reaction solvent is changed from
H2O to H2O + CH3OH, the topology of the resultant complexes
is altered from 2-D partially decarboxylated compounds 2, 8 and
11 to 0-D/1-D partially esterified compounds 3, 6 and 9,
showing the solvent also has an influence on the topology of the
final products.
Further analysis found that in the present study, the different
coordination modes of these ligands of 2,3-H2qldc, as well as its
derivatives of 2,3-Hqldc-OCH3 and 3-Hqldc are the underlying
reason behind the differences in the structure of this series of MII
complexes. At low temperature, the ligand 2,3-H2qldc mainly
adopted a simple fashion I with an NO chelating coordination
mode to give 0-D mononuclear compounds, and uncoordinated
3-position carboxylic group easily forms intermolecular hydro-
gen bonds, resulting in the formation of more stable supramole-
cular structures (for example, 1 and 4). At high temperature, the
ligand 2,3-H2qldc reacting with MCl2 in the medium of H2O
easily decarboxylated at the 2-position to give μ2-bridging ligand
3-qldc− in favor of the assembly of 2-D metal–organic com-
plexes (for instance, 2, 5 and 8). Meanwhile, because of the
existence of the big aromatic quinoline ring in the ligand 2,3-
H2qldc, the 3-D order supramolecular structure is also easily
assembled through inter-layer π⋯π stacking interactions.
Luminescent property
As reported previously, metal–organic compounds have the
ability to affect the emission wavelength and intensity of the
organic material through metal coordination.13 Therefore, it is
important to investigate the luminescent properties of metal–
organic compounds in view of their potential applications. The
luminescent behaviors of compounds 4–9 and free ligands were
investigated in the solid state at room temperature as shown in
Fig. 11. Apparently, the emission spectra of the complexes
11904 | Dalton Trans., 2012, 41, 11898–11906
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resemble that of free ligands except for the emission intensity
and peak positions, indicating that the fluorescence of the com-
plexes are ligand-based emission. Meanwhile, green emission
for the complexes and ligands is observed, where the maximum
emission wavelength is 428 nm (under 361 nm excitation) for
the free ligands 2,3-H2qldc, 408, 428 and 421 for complexes 4,
5, 6 (corresponding excitation under 359, 355 and 359 nm), and
408, 408 and 432 for complexes 7, 8, 9 (corresponding exci-
tation under 359, 355 and 359 nm), respectively. Compared with
the emission spectra of 2,3-H2qldc, a varying degree of blue
shifts of 20 nm in 4, 7 and 8, 7 nm in 6 and red shifts of 4 nm in
9 were observed, which are derived from the different topologi-
cal, supramolecular structures and different dimensions. More-
over, from Fig. 11, we find that the fluorescent intensity of
complexes 5 and 8 are stronger than that of others, and it may be
attributed to complexes 5 and 8 possessing a 2-D framework
structure while the others have a 0D or 1D structure. As for 9
with a stronger peak than 7, there is a possible explanation that 9
has a 1-D chain-like structure and can be assembled into a 3-D
supramolecular network by intra-chain stacking interactions.
Meanwhile, the enhanced fluorescence intensities of these com-
plexes are detected, which indicates that it is a good candidate
material for photochemical applications of these complexes,
especially for complexes 5 and 8.13
systematically analyze the effect of the related parameters in
order to filter out those ones that really determine the desired
network properties.
Acknowledgements
The authors are grateful for financial aid from the National
Natural Science Foundation of P. R. China (Grant Nos.
91122008 and 21071056), Science and Technology Planning
Project of Guangdong Province (Grant No. 2010B031100018)
and the N. S. F. of Guangdong Province (Grant No.
9251063101000006) and Science and information Technology
of Guangzhou Municipal (Grant No. 2011J52090019).
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This journal is © The Royal Society of Chemistry 2012